For the high-resolution measurement of distances, by way of example—nowadays in an increasingly standardized manner—laser interferometers are used. In this case, a collimated laser beam passes from a measuring instrument to a reflective target. The transmitted beam is superimposed on the beam which is reflected by the target and received in the measuring instrument. In the event of a change in distance, the intensity of the superimposed beams changes in accordance with the interference of the two beams. On the basis of the wavelength of the laser light and a number of detected intensity changes counted by means of a counter, it is thus possible to determine a relative distance change with respect to the target. Proceeding from a known reference value, that is to say an absolute distance in a starting position, it is thus also possible to ascertain an absolute distance from other positions.
However, such a method for determining a relative distance necessitates that the beam not be interrupted between measuring instrument and target. If this occurs, then distance changes are no longer detected during the interruption, and the absolute distance between measuring instrument and target is no longer known and must therefore be determined or calibrated anew using other means.
Various methods are known for measuring absolute distances, for example various variants based on the principle of the so-called gearwheel method according to Fizeau. In this case, originally a light beam was interrupted periodically by a gearwheel, then transmitted to a reflector and finally interrupted periodically a second time at the gearwheel. From the rotational speed of the gearwheel in the case of an extinction of the returning light beam, it is possible to determine the propagation time thereof by comparison with the propagation time of a gearwheel tooth up to the closest tooth gap.
Nowadays, instead of the gearwheel, by way of example, an electro-optical crystal is used as a modulator. In this case, the measurement beam is no longer interrupted periodically, but rather is modulated by the modulator. During the modulation, by way of example, a polarization and/or the intensity and/or the frequency of the transmission radiation are/is modulated, wherein the absolute distance to the target can be derived on the basis of the modulation phase of the returning transmission radiation.
By way of example, a measurement signal is detected by means of modulation/demodulation of at least part of the modulated transmission radiation and part of the transmission radiation returning from the target in such a way that the light intensity of the measurement signal changes periodically as a function of the reflector distance and the modulation frequency. By way of example, if the modulation frequency is increased continuously over a defined frequency range, then equidistant minima with the spacing of the wavelengths of the modulated frequency form during a synchronous recording using an intensity detector. In this case, the arising of the individual minima is based on the fact that at the individual minima at every moment there are precisely a whole number of modulation wavelengths at the doubled measurement distance between modulator and target. The measurement distance is thus essentially given by how far apart from one another two minima are in terms of frequency.
By way of example, if one minimum is at a first frequency, then the doubled measurement distance contains a whole number of wavelengths of the first frequency. If the modulation frequency is then increased continuously, at the nearest neighboring minimum of a second frequency the doubled measurement distance contains exactly one more wavelength of the second frequency. Since the same distance is measured at both frequencies, the absolute distance to the target can then thus be ascertained by measurement (location of the minimum in terms of frequency) of at least two minima.
For achieving high measurement accuracies, for example accuracies of less than 100 μm, in particular less than 1 μm, the ascertainment of the individual minima is made more difficult by atmospheric turbulences and variations of the refractive index in the measurement path. However, the atmospheric fluctuations can be compensated for by, for example, correlation with a reference signal, for example using a wobble generator for an additional frequency modulation (wobble) of the (fundamental) modulation frequency and using a lock-in amplifier for the detection of the modulated signal.
In this case, measuring instruments from the prior art use for example the sign of the correlation for the determination of the offset direction with respect to the minimum, that is to say for the determination of whether a currently set fundamental modulation frequency is below or above the frequency of the minimum point. It is true that a vague distance (offset) in terms of frequency with respect to the minimum point can furthermore also be predicted as a trend by way of the value of the correlation. However, an accurate prediction of the offset with respect to the minimum point is made impossible in practice by various factors, for example the type of reflector used, the contamination and tilting of the reflector, the targeting accuracy, the distance to the target, the laser power, or the air damping.
In the static case, that is to say for a target at a fixed distance, the minimum point can be determined for example by means of an iterative approximation—with the aid of the offset direction—and the fact that the offset with respect to the minimum point does not emerge directly from the correlation is normally of no importance. The distance measurement simply takes somewhat longer.
For signal amplification and/or elimination of measurement noise, a plurality of measurement values are typically filtered, for example by integration or averaging.
In contrast to the measurement of relative distances (for example by means of interferometry), a measurement of absolute distances according to the above method, owing to the measurement principle used, for example depending on the measurement distance (signal strength) and the accuracy (atmospheric turbulences), requires a specific minimum measurement duration during which the distance is not permitted to change. This limits for example the application of the method for a measurement and tracking of dynamic targets (targets at a radial movement velocity in relation to the measuring instrument). Therefore, in distance measuring instruments, absolute distance measuring methods, for example according to the Fizeau principle, are often combined with relative distance measuring methods, since the latter can have comparatively high measurement dynamic ranges, for example methods based on relative interferometry measurements or based on the pulse time-of-flight method.